As it is known, gas turbine electric energy production plants normally comprise a motor assembly (turbo assembly), to which belong a variable geometry stage compressor, a combustion chamber, a gas turbine and a generator, mechanically connected to the same turbine and compressor shaft and connected to an electric distribution network through a main switch.
Turbo-gas plants are further equipped with control devices, which implement the various operations needed for an appropriate plant operation and for meeting the standard requirements related to the performances of plants in terms of safety, stability and capacity of responding to variations in the demand for power by the distribution network.
Normally, when connected to the electric network, the plant outputs an electric power at a frequency which is stably maintained by control devices about a given frequency value, named nominal frequency (50-60 Hz).
Specifically, the known control devices perform the so-called primary setting, which stabilizes the plant frequency by varying the supply of fuel to the combustion chamber according to the difference between the nominal frequency and the plant frequency. The primary setting generally implements a proportional control logic.
However, the primary setting is not always sufficient to guarantee the stability of the frequency of the delivered electric power.
The plant is usually connected to a network comprising a plurality of electric energy production plants and loads, organized in a grid structure. In ordinary conditions, all the plants connected to the network participate to the frequency setting, which is stabile and subjected only to modest fluctuations. According to the diverse operation needs, portions of the network, including one or more plants, may be selectively isolated, e.g. to prevent the propagation of possible faults.
However, major frequency variations which the primary setting cannot compensate may occur in an isolated plant, especially because the isolation of the plant intrinsically implies evident imbalances between the power delivered by the plant and the power consumed by the loads. Specifically, the known control devices are not always capable of re-establishing a condition of balance (delivered power=consumed power) and thus of reaching the nominal frequency value again.